The present invention relates to a semiconductor controlled rectifier having a semiconductor body with at least four zones of alternatingly opposite conductivity types. More particularly, the present invention relates to a semiconductor controlled rectifier of the above-mentioned type wherein at least one of the weakly doped inner zones which is adjacent the outer highly doped emitter zone extends through this emitter zone to its outer surface and is directly connected with a conductive coating disposed on this outer surface which serves as a contact electrode.
When semiconductor controlled rectifiers, i.e. so-called thyristors, are used with voltages of a higher frequency than the conventional line frequency, particular importance is placed on the permissible rate of increase of the voltage dv/dt, in addition to the other dynamic properties of these components.
If a thyristor is charged in the forward direction with an increasing voltage, firing, i.e. switching from the nonconductive to the conductive state, may occur, depending on the slope of the voltage curve and the value of the voltage, even before reaching the so-called flip voltage. This phenomenon, which is undesirable in the practical use of these components can be explained in light of the known fact that the two inner highly resistant layers which form the center pn-junction, which is blocking in the forward direction, constitute a voltage dependent capacitance together with their space charge zone. This capacitance may produce an additional shifted current to flow in addition to the static blocking current. The resulting shifted current may be high enough to cause switching of the thyristor. Consequently, such an uncontrolled firing at a steeply increasing voltage occurring in the forward direction has to be prevented by the appropriate reduction of this shifted current.
In 1959, R. W. Aldrich and N. Holonyak, Jr., proposed the use of a four-layer sequence with so-called "shorted emitters" (Journal of Applied Physics, Volume 30, No. 11, November, 1959, page 1819). The metallic electrode which is provided on the emitter surface for contacting the emitter, extends beyond the emitter surface and is also connected with the adjacent p-conductive zone. Such a structure which short-circuits the emitter produces the result that when an increasing voltage is applied in the forward direction, a portion of the majority charge carriers flowing out of the p-conductive zone toward the emitter is drawn directly toward the cathode terminal and can no longer contribute to the injection of minority charge carriers from the emitter zone into the adjacent p-conductive zone and thus to the occurrence of the undesirable switching. Increasing the surface area of the p-conductive zone intended for short-circuiting the emitter leads to an increase in the permissible rate of increase of the voltage dv/dt. The surface area cannot be arbitrarily raised, however, due to the mutual arrangement of emitter, p-conductive zone and the contact electrodes disposed on the surface which arrangement is based on various considerations, and due to the requirement for optimum space utilization for a high current load carrying capability.
Embodiments of thyristors with a shorted emitter are also known in which the inner p-conductive zone which borders the emitter zone extends up to the outer surface of the emitter zone through a plurality of separate channels. Each of these channels has a small cross-sectional surface and preferably extends perpendicularly through the emitter zone. This inner p-conductive zone is then directly connected at this outer surface of the emitter zone with the metallic coating serving as the contact electrode. The total surface area indicated as the shorted emitter surface which is determined by the number and respective cross section of these channels, however, reduces the surface area of the emitter zone and thus the current carrying capability in its operation. The surface area of the emitter zone is limited therefore, by this parameter. It has also been found that the dv/dt values do not increase at the same rate as the increase in the shorted emitter surface area. The explanation for this appears to be that the majority charge carriers flowing out of the p-conductive zone in the direction of the emitter zone do not pass in the desired manner through the channels of the known structure toward the contact electrode because each one of these channels, with its narrow cross section as compared to the surrounding emitter surface, determines the intake region for drawing in the charge carriers. Furthermore, increasing the shorted emitter surface area appears to influence the pn-junctions disposed between the emitter zone and the channels of the p-conductive zone.